If you bulk edit all your listings, those without a Country of Origin are highlighted as soon as you are taken to the bulk edit page.
It's a futer to add the Country of Origin in the bulk editing, but it can be done. Hopefully the penny will drop with eBay soon.
It doesn't just apply to GSP, we post internationally and don't use GDP. Unless the column is filled in, listings are not visible internationally.
Were finding the Royal Mail PDDP service excellent, including having tracking and notice of arrival for International Standard as well as International Tracked postage options.